The United States maintains a complex network of law enforcement agencies at the local, state and federal levels. These agencies have varying degrees of specialty and work closely with one another and the courts. Functions performed by the levels of police agencies in the United States run a gamut, from preventing the importation of illegal drugs to ensuring the safety of students at universities. The role of a police agency depends upon its location and jurisdiction.
State and Local Police Agencies
According to Institute for Global Security Studies criminal justice professor Tom O'Connor, as many as 23,000 local and state police agencies exist in the United States as of 2011, depending upon the method of counting them. Types of police agencies under state jurisdiction include local township or city police forces, state police, countywide sheriff’s offices, state highway patrols and constables. The structure and levels of state police agencies differs in each state. Hawaii, for instance, maintains no state police force per se, but rather a Department of Public Safety. In most states, sheriffs serve as elected officials and act as political liaisons between state governments and police agencies.
Specialized State Police Agencies and Units
Various agencies perform specialized police functions at state level. Fish and game wardens, for instance, enforce hunting, fishing, boating laws. According to O'Connor, 35 U.S. states invest independent agencies such as the Department of Criminal Investigations, Department of Motor Vehicles and Alcohol Beverage Control with special, limited powers of law enforcement and investigation. These agencies enforce laws within their jurisdiction, often with the cooperation of other police agencies. Other special divisions of police agencies include K-9 units, which work with police dogs, HAZMAT units, which handle hazardous materials and SWAT, or special tactical units.
Federal Police Agencies
The federal government maintains various law enforcement agencies with the goal of dealing with problems extending across state lines and affecting regions of the country, if not the entirety of the United States. Federal law enforcement agencies include the FBI, Drug Enforcement Administration, Bureau of Alcohol, Tobacco, Firearms and Explosives, and Border Patrol. These agencies deal with issues such as immigration, drug sales and importation, the regulation of weapons, and threats to domestic security such as terrorism. Federal law enforcement agencies have jurisdiction over local and state agencies.
The Evolution of Police Agencies
Police forces initially arose in the colonial United States as a means of social control, subduing American Indians through violence, and monitoring slaves to prevent escapes or revolts. U.S. police agencies evolved based on the English model, with ordinary citizens initially serving as constables and night watchmen. Until the 1830s, cities maintained no police force other than night watchmen, but growing crime problems precipitated the necessity for crime prevention special units. By 1861, crime problems led to the creation of special crime-fighting police forces in many major cities. The evolution of social problems and crime in the United States coincided with the development of the levels of police agencies -- the DEA arose from an increased focus on illegal drugs while the Department of Homeland Security arose from the Sept. 11, 2011, attacks.
More Police Agencies
Large public universities such as the University of Massachusetts, Amherst often maintain police forces on campus. Several private police forces exist throughout the United States, policing places such as gated communities and nightclubs. These forces often consist of former or current police officers. Private security companies such as the firm formerly known as Blackwater Worldwide worked on government contracts in war zones in Iraq and Afghanistan during the 2000s.